Silicon carbide (SiC) is a chemically highly-stable material having a wide band gap of 3 eV, and can be used extremely stably as a semiconductor even at high temperatures. The highest field intensity of silicon carbide is 10 fold higher than that of silicon (Si) and therefore, silicon carbide attracts attention as a material to replace silicon, which is currently at its performance limit, from the viewpoint of power semiconductor devices (see, e.g., Non-Patent Literature 1 below).
For a semiconductor apparatus, in the last stage of the fabrication process, a metal wiring film is formed to connect the semiconductor apparatus to an external apparatus. Many conditions are required for this metal wiring film such as low contact resistance, no peeling during dicing, and durability of the bonding/die bonding and the long term use after the bonding without any peeling. However, no peeling is especially required and the high cohesion is required. A silicon carbide semiconductor apparatus using silicon carbide as its material is no exception to the above.
A silicon carbide semiconductor apparatus using silicon carbide as its material tends to have a layer formed on a surface that induces peeling off such as a graphite layer because the silicon carbide includes carbon; many high temperature process steps are used to cause carbon to react with a metal in the process of fabricating (manufacturing) the semiconductor apparatus; and many process steps need to further be executed after the reactions. Therefore, the metal film for wiring, etc. is deposited on the layer inducing the peeling such as the graphite layer and therefore, the metal wiring film tends to peel off.
Noting especially the process of forming an ohmic electrode to establish a low resistance connection in the silicon carbide semiconductor apparatus, it has been reported that merely, a nickel (Ni) film has to be deposited on a silicon carbide substrate; and thereafter, for example, an Ni silicide film has to be formed on the silicon carbide substrate by executing a heat treatment for the deposited Ni film and the silicon carbide substrate to cause Ni in the Ni film to react with silicon in the silicon carbide substrate. However, when the heat treatment is executed for Ni to form the Ni silicide film, Ni does not react with carbon (C) in the silicon carbide substrate and therefore, the surplus carbon forms the graphite layer on the Ni silicide film. Because plural process steps are executed thereafter, contaminants degrading the cohesion are deposited on the ohmic electrode surface. When the metal wiring film for the connection to an external apparatus is fabricated on a surface having the contaminants deposited thereon, the cohesion therebetween is degraded and this becomes a factor to cause the metal wiring film to peel off.
Therefore, methods have been proposed of suppressing the peeling of the metal wiring film based on prevention of the peeling of the metal wiring film by removing the graphite layer formed on the surface of the Ni silicide film using a plasma process in an oxygen gas (O2) atmosphere or an inert gas (argon (Ar), etc.) atmosphere (see, e.g., Patent Document 1 below); a method of causing the graphite layer to remain under the surface by forming an Ni silicide film on an Ni film in advance before heat treatment is executed for the Ni film deposited on the silicon carbide substrate (see, e.g., Patent Document 2); and a method of depositing a metal reactive with carbon on an Ni film, executing heat treatment therefor, and thereby, forming a metal carbide layer on the surface of the Ni film (see, e.g., Patent Document 3).    Patent Document 1: Japanese Laid-open Patent Publication No. 2003-243323    Patent Document 2: Japanese Laid-open Patent Publication No. 2006-332358    Patent Document 3: Japanese Laid-open Patent Publication No. 2006-344688
IEEE Transactions On Electron Devices (Vol. 36, p. 1811, 1989)